A new 5G radio access technology (RAT), which is envisioned to be operated in both legacy cellular bands and frequency bands above 6 GHz, is likely to exploit advanced Multiple Input Multiple Output (MIMO) (e.g. massive MIMO), Cooperative Multi-point (CoMP) transmission and reception schemes, and multi-connectivity, in order to provide high area traffic capacity and consistent user experience. Narrow beam based system operation with a large number of antennas may increase spectral efficiency by potentially reducing the interference and enabling more users to be spatially multiplexed. Beamforming is a main technology component to enable operation in mid-to-high frequency bands.
In the conventional cellular system, a UE (User Equipment) is typically attempting to connect with a single cell or a single access point (AP) at a time during a random access procedure. After a Radio Resource Control (RRC) connection being established with primary cell (PCell) or master enhanced Nobe B (MeNB)/master cell group (MCG), the UE may be directed by the PCell or MeNB/MCG to further perform some contention free random access with a secondary cell (SCell) or secondary eNB (SeNB) for carrier aggregation or multi-connectivity operation, respectively. These procedures may be applied directly to new 5G RAT technologies for multi-connectivity.